Activated carbon is commonly used in the water industry for the removal of a variety of contaminants. Such contaminants include, for example: chlorinated, halogenated organic compounds (such as trihalomethanes), adsorbable organic halogens (AOX), odorous materials, coloured contaminants, compounds for biological treatment systems, aromatics, pesticides, etc. Unfortunately, irrespective of the precursor source or whether the activated carbon is virginal or reactivated, activated carbon imparts an alkaline character to water upon contact. As a result, the pH of the effluent can rise to a value exceeding 9 or 10. This excursion in alkalinity, commonly referred to as a pH spike, can result in the leaching of aluminium from the activated carbon and, additionally, the leaching of manganese and other transition metals from reactivated carbon. The net effect of this increased alkalinity is that large quantities of high-pH water are wasted by the need for excessive backwashing/extraction of the carbon in order to bring the pH back to within the potable range. This remedial activity can last for several days, sometimes requiring as many as 800 bed volumes of water. Considering that water beds used in water treatment plants generally have capacities of 2 to 50 cubic meters, remediation can require a significant volume of water.
U.S. Pat. No. 5,876,607 assigned to Calgon Carbon Corporation describes a method for treating water to control pH and aluminium concentration in the water using activated carbon (exemplified by F400) soaked with water then treated with either carbon dioxide or carbon dioxide followed by air. Such use of carbon is occasionally employed, but has not become part of common industrial practice owing to the high costs involved in draining and disposal of the initial soak water necessary to wet the carbon in preparation for carbon dioxide treatment. Additionally, the transportation burden of the water wetted carbon, and even then the continued need for a number of bed-washes to stabilize the waters' pH has prevented common use of the method.
Thus, there is a need for a more effective and efficient process for treating water with activated carbon that reduces any excessive pH rise and consequent increase of metal ion concentration in water, and a process that overcomes the shortcomings of the prior art. It is also desirable to provide a satisfactory means of making efficient contact of the carbon dioxide with the activated carbon to be used for the water treatment application.